(i) Field of the Invention
The present invention relates to a hydrogen separator for separating a hydrogen gas only from a mixed gas containing hydrogen, a hydrogen separating apparatus using this hydrogen separator, and a method for manufacturing the hydrogen separator.
(ii) Description of the Related Art
A hydrogen gas has been used in large quantities as a fundamental material gas in a petroleum chemistry, and much expectation is put on the hydrogen gas as a clean energy source. The high-purity hydrogen gas can be obtained by converting a natural gas, a naphtha or the like as a material into a gas containing hydrogen by virtue of a catalyst, and then separating the hydrogen gas from the hydrogen-containing gas.
The hydrogen gas can be separated by utilizing the characteristics of the hydrogen gas that the hydrogen gas can be dissolved in palladium or an alloy containing palladium. Since the hydrogen gas alone can be dissolved in these metals, the hydrogen gas can be selectively separated.
In the case that palladium or the alloy containing palladium is used as a hydrogen separator, it is usually formed into a thin film. However, when the palladium thin film is singly used, its mechanical strength is poor, and thus in Japanese Patent Application Laid-open No. 273030/1987, a porous substrate of a porous ceramic or the like is coated with the palladium thin film to increase the mechanical strength.
In general, a hydrogen permeation velocity in the palladium film or the palladium alloy film can be represented by the formula (1) EQU Q=S/t(P.sub.1.sup. 1/2 -P.sub.2.sup. 1/2)K (1)
wherein
Q is a hydrogen gas permeation velocity (Ncm.sup.3 /min), PA1 S is a film area (cm.sup.2), PA1 t is a film thickness (cm), PA1 P.sub.1 is a partial pressure of a hydrogen gas in a material gas (kg/cm.sup.2 abs), PA1 P.sub.2 is a partial pressure of a hydrogen gas in a permeated gas (kg/cm.sup.2 abs), and PA1 K is a hydrogen gas permeation velocity constant [Ncm.sup.3 /min.multidot.(kg/cm.sup.2).sup. 1/2 ].
As understood from the above-mentioned relation, in order to increase the permeation velocity of the hydrogen gas, it is necessary to increase a difference between the hydrogen gas partial pressure in the material gas and the hydrogen gas partial pressure in the permeated gas. Therefore, when a reformed gas containing methane, carbon dioxide and the like is used as the material gas, the material gas pressure is set to a high pressure of from several kg/cm.sup.2 abs to about 10 kg/cm.sup.2 abs, and the permeated gas pressure is set to a pressure of from negative pressure to several kg/cm.sup.2 abs.
As a technique of coating the porous substrate with the palladium thin film, there has been known a chemical plating method. For example, Japanese Patent Application Laid-open No. 4216/1989 has disclosed a technique which comprises subjecting a substrate of a porous ceramic to electroless palladium plating, and then forming a plating layer of electrolytic palladium or a palladium-containing alloy thereon.
Furthermore, in Japanese Patent Application Laid-open No. 164419/1989, it has been disclosed that a palladium thin film is formed on the surface of a heat-resistant porous substrate and a silver thin film is further formed on the palladium thin film by the chemical plating method, respectively, followed by a heat treatment, to allow silver and palladium to interdiffuse, thereby forming a hydrogen separating film comprised of an alloy of silver and palladium.
In the conventional hydrogen separators, however, the shape of the porous substrates is tubular or planar, and so it is difficult to increase the area of the hydrogen separating films in a certain volume so as to heighten a hydrogen separation efficiency (a volume efficiency) per unit volume.
For example, if it is intended to increase the volume efficiency by the use of the tubular hydrogen separators, the respective tubular hydrogen separators must be thinned so that the most possible hydrogen separators may be received in a certain volume. However, if the tubular hydrogen separators are thinned, the strength of these separators unavoidably deteriorates. In addition, it is difficult to manufacture a plurality of the tubes having the same size with a good accuracy, and even the slight unevenness of the size of the tubes makes the integration of these tubes difficult sometimes.
Moreover, in the conventional hydrogen separators, the hydrogen separating films are often formed on the outer surfaces of the porous substrates. However, most of the porous substrates are lower in thermal expansion coefficient as compared with palladium or an alloy containing palladium, and for this reason, when the porous substrates are used at a high temperature, the hydrogen separating films with which the outer surfaces of the tubular porous substrates are coated tend to peel and crack, so that the material gas might leak into a purified gas.
When the hydrogen separators are attached to flanges or the like to construct the hydrogen separating apparatus, or when this apparatus is used, vibration occurs, so that mechanical shock or friction is applied to the outer surfaces of the hydrogen separators to damage the hydrogen separating films on occasion.
In the case that a difference of temperature distribution occurs in the hydrogen separating apparatus in which a plurality of the tubular hydrogen separators are used, the conduction of heat is not carried out between the hydrogen separators, so that thermal expansion increases in part of the hydrogen separators, with the result that high stress is generated at the bonding positions of the hydrogen separators and the flanges. In consequence, airtightness at the bonding positions is inconveniently impaired.